The only reason bermuda thins out is wear and tear or too much shade. You have one or the other. If you have too much shade, try another ground cover that tolerates shade or move the flower bed out farther.

This is exactly what it looks like at the base of a fence (east, west, and north sides), against a building, or under dense foliage (trees or shrubs).

I really think the amount of sun is fine - however, there was a lot of wear and tear from foot traffic during deck construction two summers ago. The spots have SLOWLY gotten smaller. If I were to temporarily corden off the areas so no one could walk on them at all.....then should I amend, fertilize, sand, compost....anything? or just let nature take it course?

What really helped for the bare spots in my bermuda, believe it or not, was using a hand cultivator and plugger. If the bare spots are pretty large like yours, I would try both cultivating and plugging. Bermuda runners don't seem to like clay at all, so you may be surprised how tilling the soil will help. Here is the plugger I bought:

https://www.amazon.com/ProPlugger-Garden-Planter-Weeder-Plugger/dp/B003MRTVUI?keywords=proplugger&qid=1539637412&sr=8-1&ref=sr_1_1

Here is the type of cultivator (bought mine at Lowe's):

https://www.amazon.com/Garden-Weasel-Cultivator-Long-Handle/dp/B002ECYRH4?crid=1XDWO71220E5Q&keywords=soil+cultivator+tool&qid=1539637589&sprefix=soil+cul%2Caps%2C196&sr=8-8&ref=sr_1_8

First cultivate the area, then find a nice thick patch of bermuda, that you think will fill back in quickly in full sun. Transplant a dozen or so plugs to the bare spots, and fill the holes back in with good garden soil. As long as you have enough hours of sunlight you should have the area fully filled in within a couple months.

Good luck!